The present invention relates to transducers that utilize induced strain as a means for measuring acceleration, pressure, temperature and other variables, and more particularly to transducers employing resonant beams as sensors.
Resonant sensors have been used for many years to achieve high accuracy measurements. Vibrating transducers have been used in accelerometers, pressure transducers, mass flow sensors, temperature and humidity sensors, air density sensors and scales. These sensors operate on the principle that the natural frequency of vibration (i.e. resonant frequency of an oscillating beam or other member) is a function of the induced strain along the member. More particularly, tensile forces tending to elongate the beam increase its resonant frequency, while forces tending to compress the beam reduce the natural frequency. The frequency output of resonant gauges is readily converted to digital readings reflecting the measured quantity, requiring only a counter and a reference clock for this purpose. Thus, such gauges are simple and reliable, providing a high degree of discrimination while using a relatively simple sensor to digital interface.
An exemplary use of a vibrating beam transducer is shown in U.S. Pat. No. 3,486,383 (Riordan). A pair of parallel beams are employed to limit the angular movement of the gimbal of a gyro. Angular movement in one direction tends to compress the vibrating beams, while angular movement in the opposite direction tends to place the beams under tension. Changes in natural frequency of the beams provide a direct indication of gimbal angular movement.
U.S. Pat. No. 5,090,254 (Guckel et al) discloses a resonant beam transducer including a polysilicon beam mounted to a substrate for vibration relative to the substrate, and a polysilicon shell surrounding the beam and affixed to the substrate to form a cavity which is sealed and evacuated. The beam is oscillated by supplying an oscillating voltage to an electrode on the shell.
U.S. Pat. No. 3,657,667 (Nishikubo et al) discloses a mechanical vibrator having three parallel arms and three piezoelectric elements, one glued to each of the arms. The element on one of the outer arms is used to drive the vibrator, while the remaining piezoelectric elements provide a pair of sensors. The sensors provide an input to an amplifier, with the output of the amplifier being provided to the drive piezoelectric element.
Resonating members also have been driven magnetically. In U.S. Pat. No. 4,801,897 (Flecken), a magnet is mounted to each of two parallel fluid carrying tubes. A coil magnet, positioned between the two tube magnets, is actuated by an excitation circuit to oscillate the tubes. Optical sensors determine the positions of the oscillating tubes, and provide position information as input to the excitation circuit.
A dual vibrating beam force transducer is shown in U.S. Pat. No. 4,901,586 (Blake et al). A pair of parallel beams are positioned between a pair of electrodes. A drive circuit provides an oscillating voltage to the electrodes to electrostatically drive the beams, causing the beams to oscillate in a plane containing both beams. The mechanical resonance of the beams controls the oscillation frequency. In an alternative embodiment (shown in Blake's FIG. 7), one of two parallel beams is grounded while a drive circuit applies an oscillating voltage to the other beam, thus electrostatically oscillating both beams.
One of the primary advantages of resonant gauges is that the resonant frequency depends only on the geometrical and mechanical properties of the oscillating beam, and is virtually independent of electrical properties. As a result, precise values (e.g. resistance and capacitance) of drive and sense electrodes are not critical. A possible disadvantage is that any parasitic coupling between the drive and sense electrodes may diminish accuracy of the resonant gauge. Furthermore, in a conventional capacitive drive arrangement, the force between the oscillating beam and drive electrode is quadratic, resulting in an unwanted frequency pulling effect. While crystalline quartz piezoresistors have been satisfactorily employed in resonant gauge applications, their size limits their practical utility.
Therefore, it is an object of the present invention to provide a resonant beam sensing device in which drive electrodes and sense electrodes are isolated from one another in a manner to virtually eliminate parasitic capacitance between them.
Another object of the invention is to provide a resonant gauge in which the force applied to the drive electrode is linear rather than quadratic, whereby the applied force can be varied in a manner that more closely approximates the behavior of the oscillating beam.
A further object of the invention is to provide a resonant beam strain sensing device with a high degree of discrimination for accurately sensing even slight changes in resonant frequency.
Yet another object is to provide a resonant gauge of microscopic dimensions for use in applications where space is severely limited.